An insulating foam layer of a high frequency coaxial cable is requested to have not only superior electric properties, such as dielectric constant and dielectric tangent, but also a certain level of mechanical strength that prevents deformation and buckling when an external force is applied to this cable.
Conventionally, an insulating foam layer of a high frequency coaxial cable has been produced by mixing a nucleator with a low polarity organic polymer, such as polyethylene, as a foamable material and extruding the mixture in the presence of a foaming agent. As the nucleator to be used for this extrusion foam forming, so-called chemical foaming agents such as 4,4'-oxybisbenzenesulfonylhydrazide (OBSH), azodicarbonamide (ADCA) and the like, and boron nitride have been used.
OBSH and ADCA are decomposed by the high temperature heat in an extruder and form a number of fine nuclei in an organic polymer, which cause uniform foaming of the organic polymer. However, OBSH produces water upon thermal decomposition to degrade electric properties of the insulating foam layer, and ADCA also forms various high polarity decomposition residues upon thermal decomposition, again degrading electric properties of the insulating foam layer.
In contrast, boron nitride is superior in electric properties and heat resistance, and is free of problems associated with OBSH and ADCA. On the other hand, boron nitride is expensive and makes the foam produced therewith more expensive than conventional products. In addition, its property as a nucleator is not necessarily sufficient. To be specific, the foams obtained therefrom generally have greater cell diameter, and unsatisfactory dispersibility of the cells (uniformity of foaming). In general terms, foams having cells having a greater diameter and less uniformity of foaming have less mechanical strength and are susceptible to deformation. This has a consequence that a cable having an insulating foam layer often suffers from buckling of the insulating foam layer upon bending.
In an attempt to overcome various problems associated with the conventional nucleators, the present inventors proposed production of a high frequency coaxial cable having a polyethylene insulating foam layer using a fluororesin powder having an average particle size of 0.1-100 .mu.m as a nucleator (JP-A 9-55120). Inasmuch as a fluororesin powder is, like boron nitride, superior in electric properties and heat resistance, it is free of the problems OBSH and ADCA are suffering from. In addition, it is strikingly more economical than boron nitride, so that a high frequency coaxial cable having a foamed thermoplastic resin insulating layer is produced at a low cost.
It is therefore an object of the present invention to provide a nucleator for foaming, which improves expansion ratio, fineness of the cell and the like, and which, by the use of a small amount thereof, is capable of producing a foam made from an organic polymer and improved in fineness of the cells, uniform foaming and the like, a foamable composition comprising this nucleator, a foam obtained by the use of this nucleator and a production method of his foam.